The invention relates to a process and equipment for enlargement of colour-balanced colour prints in additive systems, where in course of correction filtering, colour filters are arranged in the path of the beam of light of the objective of the enlarger beneath the lens, where they are alternately actuated. At the same time the time of exposure needed for obtaining the colour-balanced picture is determined by means of a trial picture. The colour control and integrating equipment, i.e. the outfit for the realization of the process and forming an object of the invention, contains additive colour filters and an integrating plate which is to be held under the objective.
A colour-analyser mask is employed as a part of the process and also forming an object of the invention, contains additive colour filters and a grey wedge.
By using the process and the equipment according to the invention traditional enlargers may be rendered suitable for producing colour-balanced enlargements, as well as simultaneously determination of the proper time of exposure.
According to the prior art equipment suitable for coloured enlargements are based mostly on the subtractive process. Complete enlargers being suitable for coloured enlargements based on the additive method have been produced in some countries (e.g. USA, GFR, Italy, etc.). However, these are so expensive that the purchase thereof for the majority of amateurs becomes simply impossible, at least it is far too uneconomical. High prices result first of all from the fact that electronic means and dichroic colour filters are built-in to such equipment to an increasing extent. The same stands for the enlargers based on the subtractive system and the colour analysers, which can be separately purchased.
For a majority of the amateurs the only possibility so far offered by the market is to supplement their traditional black-and-white enlargers with colour filtering means needed for the coloured enlargement. However, in course of coloured development the most difficult task is to learn the so-called colour filtering, representing an essential condition for obtaining colour-balanced pictures, requiring--according to our experiences--extensive theoretical knowledge and long practice. These requirements, however, surpass the possibilities of an average amateur or hobby-photographer.
Complementary instruments, such as a colour-analyser and the expensive electronic enlargers have worked well in professional developing installations, but do not offer help for the amateurs for home-work. The prerequisite for reliable operation of such equipment lies in the maintenance of the technological processes on a controlled, continuous and stable level. This requirement cannot be met in the domestic laboratories of the amateurs, being the bathroom in a majority of cases. Any change in the factors of development leads to colour-shift and there is not a single electronical device which could evaluate these phenomena in advance.
After having recognized the difficulties, some experiments were already performed in the fifties, to establish a process based on the additive system, by using either the so-called colour wedge which could be produced by crossing three (blue, green and red) colour filters and a photographic grey wedge or by using colour filters which had gradually darkening colour density. The essence of this method is, that the colours of the negative picture are integrated to grey by the aid of a diffuser and are projected on to the photographic paper, while the times of exposure suitable for adjusting the colour balance had to be determined by recognizing the faintest grade of the colour steps of the paper picture. This theory has been adapted and developed, in so far as graphic markings were used to promote the perception of the "faintest" colour, being utmost subjective in regard to judgement. Such a method is described in the British patent specification No. 1 341 288 (Exposure Calculator and Filter Device for Darkroom Color Photography).
Experiments are also known, which aimed--taking the proposals of the contemporary technical literature as a basis--to fasten the additive colour filters to the lens of the enlarger. This method has been adapted by the cited British patent but the solution was applied by other apparatuses too; as an example the additive filter-set "Tricolor" of SIMMARD.
However, the solutions mentioned involved serious defects and errors. The most decisive error was represented by the basic principle itself, which was maintained as a common characteristic of all the methods having been used up to now, despite their differing versions; namely the recognition of the faintest shades of colours. Any of the developed photographic materials shows the characteristic feature, in so far that the density of the negative is proportional to the quantity of the incident light but within certain limits, in the straight section of the so-called density curve. In the initial slowly ascending part of the curve, just where limit darkening of the negative becomes visible, density follows the change of light intensity far slower, than in the larger or straight section of the curve. This regular phenomenon is accompanied by the practical consequence, that the beginning of the limit-blackening recognized by the eye, i.e. the faintest shade hardly changes in dependence of even a considerable increase of exposition, although such differences in exposition are sufficient to already disturb the colour-balance. Furtheron, this method could be applied only where the effective shades of colour of the yellow, magenta and cyan colour grades of the photographic paper are in an accurate mutual distance within the colour cycle and the same quantities, i.e. were the same limit values result in the grey colour. However, in practice this is impossible. Sometimes the factories producing photographic materials use colouring agents having a quite different character and considerably differing wavelength than at other times. As a consequence, this method has proved erroneous and inaccurate from several points of view. It is for this reason, that the colour wedge--being perfect in itself, as means--has been almost forgotten.
The second main disadvantage lays in the fact that the additive colour filter device was fastened to the lens of the enlarger. This solution can be considered as unsuccessful from several points of view. First of all, because the lenses are sunk in several enlargers and do not protrude from the lower supporting frame, accordingly attachment becomes impossible. Secondly, in a majority of cases the solution prevents the regulation of the diaphragm, since the clamped device blocks its path. Thirdly, the lens is rather delicate, and it is unadvisable to burden it with a foreign load and to subject it to the clamping force needed for stable fixing of a separate device.